Added flake description and nixosConfigurations.vm ouput

nixosConfigurations.vm to be used to test invoiceninja-desktop in a VM

flake.nix

@@ -1,4 +1,6 @@
 {
+    description = "Invoice Ninja desktop package.";
+
     inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-24.11";
 
     outputs =
@@ -18,6 +20,11 @@
           );
         in
         rec {
+          nixosConfigurations.vm = nixpkgs.lib.nixosSystem {
+            system = "x86_64-linux";
+            modules = [ ./vm ];
+          };
+
           overlays.default = final: prev: { invoiceninja-desktop = final.callPackage ./package.nix { }; };
 
           packages = forEachSystem (system: {